Share

When Congress convened its first meeting on the IPCC's report on climate change in 2007, Dana Rohrabacher - the Republican representative of California's 46th district - floated a rather unusual idea. Even though the report had concluded that greenhouse gases released into the atmosphere by human activity were among the chief drivers are modern-day climate change, Rohrabacher appealed to climate shifts of the ancient past to shift the blame away from Homo sapiens. There were obviously no humans around during the Paleocene-Eocene Thermal Maximum when the global temperature jumped 11°F in a period of 20,000 years starting 55.8 million years ago - an event Rohrbacher said might have been triggered by "dinosaur flatulence" - and so our species could not be held responsible for our planet's shifting climate today.

As a climate change denialist, Rohrabacher had ignored the ample scientific evidence that human activity since the Industrial Revolution has had a major influence on earth's climate. He also failed to read the fossil record correctly.If anything, by looking at the fossil record Rohrabacher should have recognized how climate change during the prehistory has shaped the ecology and evolution of many of earth's organisms, just as anthropogenic climate change is doing the same today.

Over the course of the past 65 million years, the climate was never warmer than it was during the first half of the Eocene (~56-34 million years ago). Most likely triggered by an immense release of methane - a greenhouse gas - from the deep sea, the global climate quickly warmed and allowed the spread of semitropical jungles across large swaths of the planet which host more temperate ecosystems today. Even the chilly reaches near the earth's poles were significantly warmed, and a study recently published in *Earth and Planetary Science Letters *highlights just how much has changed since the seemingly endless summer of the Eocene.

Locked in the frozen strata of northern Canada's Ellesmere Island are the vestiges of warm, 52 million-year-old swamps. Alligators, tortoises, early primates, tapirs, and other creatures representative of milder climates lived in this place, but just how warm was it? To find out, Jaelyn Eberle and colleagues turned to the teeth and bones of the fossil animals themselves to find out.

Fossil bones are not just anatomical curiosities. Looked at the right way, they can also be unique time capsules which contain clues about the biology and ecology of the animal, and one such subset of preserved data comes in the form of oxygen isotopes. Trapped in bone and tooth enamel, oxygen isotope values are indicative of the temperature of the water ingested by the animals during periods of bone growth and thus provide a proxy by which to estimate the local temperatures during the time that the fossils were preserved.

In the case of the present study, the scientists used oxygen isotope values from the semi-aquatic, herbivorous mammal Coryphodon as a proxy for the oxygen isotope values of the river water it drank. Body temperature changes can potentially influence oxygen isotope values in ecotothermic animals, but since Coryphodon was a mammal that maintained a constant body temperature the isotope values in its skeleton provided as close a look at the actual oxygen isotope values of the water as could be expected. Once the scientists had this value, it was combined in an equation with the oxygen isotope values from bowfin fish which lived in the ancient swamp. Since the fish grew constantly - and therefore were constantly incorporating oxygen isotopes into their skeletons - the isotopes in their bodies could be used to help estimate the average annual temperature. Isotope values from freshwater turtles added another kind of data. The turtles would have incorporated most of their oxygen isotopes into their bones during periods of summer-month shell growth, and therefore their oxygen isotope values would be indicative of temperatures during the warmest periods of the year.

When the scientists finished analyzing the data from the teeth and bones of the Ellesmere Island fossils, they found that the Arctic island once hosted a warm, seasonal environment. The average annual temperature was about 46°F, with an average summer temperature of about 68°F and average winter temperatures between 32°F and 38°F. Given that the average winter temperatures are almost equivalent to the average annual temperatures in the northernmost part of Ellesmere, it is clear that during the Eocene the area enjoyed a more equitable climate, though it was not quite a tropical paradise. Despite the presence of alligators there, it was still a bit cooler than the environments inhabited by alligators today, hinting that alligators of the Eocene may have been more tolerant to cooler temperatures than their living cousins. The existence of large land tortoises in these deposits might also be attributable to the greater tolerance of ancient reptiles; living tortoises can't be taken as perfect proxies for their prehistoric cousins despite their ancient appearance.

The Eocene fossils of Ellesmere Island are just one window into that peculiar period in earth history. Another type of extreme environment hundreds of miles to the south also contains remnants of a lush Eocene world; Wyoming's Bighorn Basin. When I visited this area a few months ago I quickly learned how much the temperature can change during the course of a day. Summer days are marked by baking heat but I was left shivering beneath my sheets in my cabin at night. Had I been able to go back 55.8 million years, though, I would have encountered a landscape draped in lush vegetation and inhabited by a menagerie of mammals both strange and familiar. Among them were the creodonts - archaic carnivores which were among the dominant terrestrial hunters in the days before the kin of cats and dogs became apex predators - and by tracking the size of one particular kind of creodont paleontologists have found new evidence for a strange pattern among the Bighorn Basin mammals.

One of the reasons paleontologists return to the Bighorn Basin year after year is the way in which it intricately documents the transition between the Paleocene and Eocene. There is an almost continuous record between the two epochs, and the richness of the mammalian fossil record there has allowed paleontologists to track the evolution of many species and genera over time. One of those genera is the creodont Palaeonictis, a variety of predator with heavy jaws and blunted teeth which appear to have been well-suited to bonecrushing. Several species are known - both from Europe and North America - but a newly-discovered species named Palaeonictis wingi is remarkable for its size. Compared to the preceding and succeeding species of Palaeonictis, the new species was significantly smaller, and it was hardly unique in this respect. During the time it lived, at the height of the Eocene temperature spike, many other species of mammals had become downsized before becoming larger again once the heat was off.

As hypothesized by Stephen Chester and colleagues in their description and study of Palaeonictis wingi, the evolution and dispersal of this genus of predator generally tracks the changing climate. Both P. wingi and the similarly-sized P. gigantea from Europe were probably descendants of the larger North American species P. peloria; the former stayed at home while the latter crossed the land bridge to Europe. The origin and dispersal of these smaller species coincided with the temperature spike, although it appears that P. wingi was ancestral to a larger North American species called P. occidentalis. Given that these relationships are largely based upon teeth and jaws - in some cases, a single set - they are subject to revision, but if they are accurate then the evolution of Palaeonictis acts as an indicator of changing climate.

Just why Palaeonictis became dwarfed is unclear. Temperature size, aridity, and the dwarfing of prey species may have triggered this change, but whatever the trigger was it did not affect all carnivores in the same way. The Eocene mesonychid Dissacus - another archaic carnivore often cast as being a "wolf with hooves" despite its evolutionary distance from canids - did not shrink like Palaeonictis did. Perhaps this was because Dissacus was primarily a scavenger, Chester and co-authors suggest, although there are no mammals which are obligate scavengers today and so this seems unlikely. It may even be the case that the apparent dwarfing which took place was not a straight-line reduction in size among populations within the Bighorn Basin, but actually represent the replacement of large species by smaller ones in the same area due to the increased temperatures. A wider sampling of the fossil record will be required to better test this hypothesis. What is clear, though, is that the larger species of the late Paleocene gave way to smaller species at the opening of the Eocene before rebounding to larger body sizes again. The reasons why remain mysterious for now.

As the Bighorn Basin dwarfs illustrate, the fossil record cannot always be easily read. There is the basic geological and anatomical data, but restoring ancient interactions and ecosystems is often a tricky process. Nonetheless, from Ellesmere Island to Wyoming, the mammal fossils of the early Eocene clearly show that climate change can have a major influence on the history of life on earth. Warming by a few degrees centigrade can turn frozen tundra to lush seasonal swamps haunted by alligators, and under especially extreme conditions natural selection may push some animals to become adapted in unexpected ways (such as dwarfing). This is not to say that we can expect current climate change to bring back the world of the Eocene; rather that climate change can be a powerful force in shaping the history of life on earth, and we are now driving some of that change.

Top image: The skeleton of Coryphodon, a hippolike mammal which inhabited the swamps of modern-day Ellesmere Island during the early Eocene.